1. Field of the Invention
The present invention relates to a sheet processing apparatus that performs post-processing on sheets discharged from an image forming apparatus, such as a copying machine, a printer, and a multifunction peripheral.
2. Description of the Related Art
Recently, there is an image forming apparatus of a type in which a sheet post-processing apparatus is provided adjacently to a paper discharge unit of the image forming apparatus main body to perform post-processing, such as sorting sheets and performing staple processing on sheets after images are formed thereon.
After the post-processing, such as stapling, is performed by the sheet post-processing apparatus, the sheets are discharged on a paper discharge tray. The paper discharge tray stands by at a specific height position and successively receives sheets having undergone the post-processing.
Generally, the paper discharge tray is allowed to ascend and descend, and it descends as the received sheets increase so that the top surface of the sheets on the paper discharge tray will not exceed the specific height position. Also, sheets on the paper discharge tray are aligned by causing the paper discharge tray to descend once each time the predetermined number of sheets are stacked on the paper discharge tray and ascend again.
Incidentally, the sheets stacked on the paper discharge tray hit against the wall of the housing of the sheet post-processing apparatus at their rear ends. Hence, when the paper discharge tray is ascended and descended, the sheets bend as they rub against the housing in some cases, which may possibly result in poor alignment.
The sheet post-processing apparatus is described in Japanese publication of patent applications, JP-A-2004-299911 and JP-A-2000-63028.
The sheet post-processing apparatus in each application includes plural paper discharge trays disposed one above the other to accommodate sheets discharged from the image forming apparatus. The paper discharge tray at the upper stage is allowed to ascend and descend while bringing the end fence used to align the rear ends of sheets in sync with the stacking surface of sheets. The end fence prevents sheets from being carried backward. The paper discharge tray at the lower stage has the end fence formed of the wall surface of the main body of the sheet post-processing apparatus.
The example described above, however, loses the effect of preventing the backward carrying when sheets are stacked over the height of the end fence provided to the paper discharge tray at the upper stage. In addition, because the paper discharge tray at the lower stage uses the wall surface of the main body of the sheet post-processing apparatus as a substitute for the end fence, the sheets bend as they rub against the wall surface of the main body in some cases.
The present invention provides a sheet processing apparatus that lessens rubbing of sheets even when the paper discharge tray is ascended and descended.